
  class User {
    constructor() {
      var privateX = "999";
      this.a = 13;
      this.change = function(){
        //te quan fang fa
        console.log(privateX);
      };
    }

    get x(){
      return 12;
    }

    fn(){
      console.log(this.a);
    }


  }

  var u = new User();
  console.log(Object.getOwnPropertyNames(u.__proto__));
  u.fn();
  u.change();
